Aerospace vehicles and systems are typically equipped with instruments that monitor the health of various systems by acquiring, exchanging and analyzing data, and the communication networks in such arrangements must be robust enough to withstand repetitive and potentially destructive forces and conditions, while transmitting and processing the data collected.
In recent years, there has been a trend to use optical apparatus in lieu of electrical or mechanical devices for the purpose of monitoring the health of such systems. Optical interrogation systems are integral components in health monitoring systems used in chemical, biological, temperature and pressure sensing environments. Optical sensing techniques are highly desirable for aerospace applications due to the lighter weight and EMI continuity, but to date such sensor systems have proven too costly or complicated to deploy.
Various solutions have been proposed and tried, including Fiber Bragg Grating (FBG) type sensor systems, fiber optic path sensing, piezoelectric transducers, comparative vacuum monitors, strain gauge based systems, ultrasonic systems, and visual indicators, but each has challenges.
Currently, while Fiber Bragg Grating devices are preferred in many fields of Structural Health Monitoring, such devices require careful modification of fiber optic cable to operate as well as complicated methods for determining wavelength shifts and other phenomena. Such systems also appear to be affected by temperature changes, requiring additional equipment to compensate.
Fiber optic path sensing is extremely attractive due to its simplicity (e.g., a loop of fiber), but the resulting system can become quite complex as monitoring for breakage or changes will typically require additional equipment such as Time Domain Reflectance equipment.
Piezoelectric devices tend to be quite expensive, due to the nature of the materials used in their construction. Such devices will also typically have features of other electrical based systems (susceptibility to EMI/Lightning) as well as the need for dual wire connections for each individual sensor. Further, such devices tend to be sensitive to certain frequencies, and may require a considerable amount of baseline data measurements in order to operate properly.
Comparative Vacuum Monitoring (CVM) makes use of very fine pressure cells and looks for pressure variations which signify cracks; this appears to make for a simple and affordable sensor design. However, CVM appears to require tubing and pressure systems in order to operate, and known CVM equipment appears to require the use of handheld systems in order to be used.
Strain Gauges are an older technology that looks at resistance changes. The sensors are quite simple, being in most cases just copper traces on a flexible substrate. Installing and reading such sensors accurately can be difficult however, and also has issues similar to the piezoelectric designs mentioned above.
Ultrasonic inspection is a technique currently being used, and requires installation of a field device run across structures and equipment in order to operate properly. Attempting to scale down such a system to an embedded type design would most likely result in a system very similar to a piezoelectric type system.
Visual inspection is the standard sensing method used at this time, and involves highly trained individuals inspecting and attempting to gauge failures of material, and estimating how long structures can last in service.
U.S. Pat. No. 6,691,007, issued to Haugse et al. on Feb. 10, 2004 and assigned to the assignee of the present disclosure, describes a system and method for monitoring conditions of a vehicle and generating a maintenance plan according to the monitored conditions. The patent discloses the use of conventional optical sensors, such as Fabry-Perot interferometric, long-period grating, and fiber Bragg grating sensors, none of which are small enough to permit miniaturization or diminished cost of the interrogator system.
A light-weight, miniaturized, and efficient optical interrogation apparatus is therefore needed to survey data from one or more optical sensors for the purpose of monitoring and reporting on the structural health of vehicle structures and systems.